1. Field of the Invention
The present invention relates to an image processing apparatus having a sensor unit for converting light from an object into charges and accumulating the charges and a sensor drive circuit for controlling an accumulation time of the charges in the sensor unit.
2. Related Background Art
In an image processing apparatus for processing an object such as a finger or a barcode, as long as it is within a range of hand vibrations, the longer an exposure time is, the less sensitivity of a sensor can be used and the lower may be made an illuminance of light such as an LED beam irradiated on an object. Thus, power consumption of the image processing apparatus can be reduced.
However, if the exposure time is extended, a sensor dark current increases in case of that ambient temperature is high or in case of that when the apparatus is placed on, for example, a front board of an automobile, then temperature inside the apparatus rises due to heating by the direct sunlight. That is, noises increase and signals of a finger or a barcode cannot be separated.
In addition, in an apparatus for detecting a fingerprint from light transmitted through a finger, shading occurs in a signal generated by the light transmitted through the finger. Influence of noises due to a dark current is large in a central part of the finger because a signal level decreases more in that part than in a peripheral part of the finger.